Edward,

During the "stim test," by augmenting one's physiological responses while denying (per the polygrapher's instructions) having selected the number one actually chose, one can help to create the impression in the polygrapher's mind that one is a "good responder." During the stim test, it would be counterproductive to augment physiological responses while answering questions about numbers other than the one actually selected.

With regard to "control" questions like, "Are you trustworthy?" it is important to bear in mind that the polygrapher expects that persons who are answering the relevant questions truthfully will produce stronger physiological responses to such "probable-lie control" questions than to the relevant questions: it is expected that even truthful persons will have some concerns about whether their answers to such questions are completely truthful, and that those concerns will result in physiological responses. The way to pass the "test" is to make sure that one's physiological responses while answering the "control" questions are greater than any physiological responses which may occur while answering the relevant questions.

For further reading on "control" questions, I refer you to Chapter 3 of The Lie Behind the Lie Detector and to Chapter 8 of David T. Lykken's A Tremor in the Blood: Uses and Abuses of the Lie Detector (New York: Plenum Press, 1998).